The present invention relates to a signal processing apparatus for a radar installed on a movable object such as, for instance, a vehicle, etc., and more particularly, to a distance and speed measuring method for detecting an object in the form of a target, and measuring the relative distance and relative speed of the object, as well as to a radar signal processing apparatus using such a method.
In radars installed on vehicles, etc., the distance of a target which is able to be measured thereby is generally in the range of about several m to about 200 m. As a radar system for detecting objects to be measured lying in such a range, there has often been used a well-known FMCW (Frequency Modulated Continuous Wave) method which is described for example in a book entitled xe2x80x9cIntroduction to Radar Systemsxe2x80x9d by M. I. SKOLNIK, McGRAW-HILL BOOK COMPANY, INC., (1962), a book entitled xe2x80x9cRADAR HANDBOOKxe2x80x9d by M. I. SKOLNIK, McGRAW-HILL BOOK COMPANY, INC., (1970), a book entitled xe2x80x9cRadar Technologiesxe2x80x9d compiled under the supervision of Takashi Yoshida and edited by the Japanese Electronic Information Communications Society (1989), etc.
FIG. 3 shows the frequency characteristics of respective signals relative to time in an FMCW radar.
In FIG. 3, 1 designates a transmission signal, 2 a reception signal, and 3 a beat signal. Assuming that a frequency sweep width is B; a frequency sweep time is T; the speed of light is c: a wavelength is xcex; the relative distance to a target is r; and the relative speed of the target is v, the frequency U of the beat signal 3 in the up phase and the frequency D of the beat signal in the down phase are represented by the following expression:                     U        =                                            -                                                2                  ⁢                  B                                cT                                      ⁢            r                    +                                    2              λ                        ⁢            v                                              (        1        )                                D        =                                                            2                ⁢                B                            cT                        ⁢            r                    +                                    2              λ                        ⁢            v                                              (        2        )            
From these relations, the relative distance r and the relative speed v of the target are obtained from the following expressions (5), (6) by using the results according to the subtraction and addition of the beat frequencies U and D, as shown by the following expressions (3), (4).                               D          -          U                =                                            4              ⁢              B                        cT                    ⁢          r                                    (        3        )                                          U          +          D                =                              4            λ                    ⁢          v                                    (        4        )                                r        =                              cT                          4              ⁢              B                                ⁢                      (                          D              -              U                        )                                              (        5        )                                v        =                              λ            4                    ⁢                      (                          U              +              D                        )                                              (        6        )            
Moreover, when there are (N) targets, the frequency Ui{i=Nu, Nuxe2x89xa6N}of the beat signal in the up phase and the frequency Dj{j=Nd, Ndxe2x89xa6N}of the beat signal in the down phase are obtained. Therefore, a frequency pair (Ux, Dy) is selected based on a criterion set beforehand. The relative distance and the relative speed of each target are obtained by substituting the frequency pair for the expressions (5) and (6).
For such a selection criterion, for example, peak strengths in the frequency spectrum of the beat signal may be employed. In Japanese Patent Application Laid-Open No. 5-142337, pairs are determined in order of the magnitude of strength thereof. In addition, in Japanese Patent Application Laid-Open No. 11-337635, there are used strength patterns which are obtained some different directions by scanning a beam.
These relative distance and relative speed of a target are generally measured repeatedly at preset time intervals.
However, in actuality, there arises a problem that the frequency of the beat signal measured in a time series manner is varied according to the state of reflection from a target in the form of a vehicle, the characteristics of the components of a transmit and receive device, etc., thus resulting in unstable measurements of the distance and speed of the vehicle.
As solutions for such a problem, Japanese Patent Application Laid-Open No. 5-142338, Japanese Patent Application Laid-Open No. 5-150035, Japanese Patent Application Laid-Open No. 5-249233, etc., disclose the use of information in a time series direction with respect to the frequency of the beat signal.
For instance, FIG. 4 shows the configuration of a signal processing part of a millimeter wave radar system disclosed in Japanese Patent Application Laid-Open No. 5-249233. The signal processing part 10 illustrated is provided with an A/D (Analog to Digital) conversion part 11, a frequency analysis part 12, a switching part 13, comparison parts 14, 18, reference value forming parts 15, 19, storage parts 16, 20, variation removing parts 17, 21, and a distance and speed deriving part 22.
Next, the operation will be described below. In the signal processing part 10 shown in FIG. 4, a beat signal 3 for a target is input as an analog signal, and this beat signal is converted into a digital signal by the A/D conversion part 11. In the frequency analysis part 12, frequency analysis is performed through the use of an FFT (Fast Fourier Transform), etc., and the frequency U of the beat signal in an up phase and the frequency D of the beat signal in a down phase are extracted.
These frequencies are associated through the switching part 13 with the point in time t at which they are measured. The frequency U is stored as U(t) in the storage part 16, and the frequency D is also stored as D(t) in the storage part 20.
At time point t, the reference value forming part 15 sets a reference value Uref(t) by using the past data stored in the storage part 16. For instance, the reference value Uref(t) is set according to the following expression (7) while assuming that an measurement interval is xcex94t.                               Uref          ⁡                      (            t            )                          =                                            U              ⁡                              (                                  t                  -                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                    t                                                  )                                      +                          U              ⁡                              (                                  t                  -                                                            2                      ·                      Δ                                        ⁢                                          xe2x80x83                                        ⁢                    t                                                  )                                      +            ⋯            +                          U              ⁡                              (                                  t                  -                                                            5                      ·                      Δ                                        ⁢                                          xe2x80x83                                        ⁢                    t                                                  )                                              5                                    (        7        )            
Similarly, the reference value forming part 19 sets a reference value Dref(t) by using the past data stored in the storage part 20. For instance, the reference value Dref(t) is set according to the following expression (8).                               Dref          ⁡                      (            t            )                          =                                            D              ⁡                              (                                  t                  -                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                    t                                                  )                                      +                          D              ⁡                              (                                  t                  -                                                            2                      ·                      Δ                                        ⁢                                          xe2x80x83                                        ⁢                    t                                                  )                                      +            ⋯            +                          D              ⁡                              (                                  t                  -                                                            5                      ·                      Δ                                        ⁢                                          xe2x80x83                                        ⁢                    t                                                  )                                              5                                    (        8        )            
The comparison part 14 compares the frequency U(t) of the beat signal in the up phase input thereto via the switching part 13 with the reference value Uref(t) set by the reference value forming part 15, and determines whether the frequency U(t) of the beat signal in the up phase is data without any variation. For instance, whether the relationship of the following expression (9) is satisfied for a preset allowance or allowable width Wu is used as a criterion for such a determination.
|U(t)xe2x88x92Uref(t)|xe2x89xa6Wuxe2x80x83xe2x80x83(9) 
Similarly, the comparison part 18 compares the frequency D(t) of the beat signal in the down phase input thereto via the switching part 13 with the reference value Dref(t) set by the reference value forming part 19, and determines whether the frequency D(t) of the beat signal in the down phase is data without any variation. For instance, whether the relationship of the following expression (10) is satisfied for a preset allowance or allowable width Wd.
|D(t)xe2x88x92Dref(t)xe2x89xa6Wdxe2x80x83xe2x80x83(10) 
The frequency U(t) of the beat signal in the up phase, for which the presence or absence of a variation was determined by the comparison part 14, is removed by the variation removing part 17 if determined as including a variation, whereas it is stored in the storage part 16 and input to the distance and speed deriving part 22 if determined as including no variation.
Similarly, the frequency D(t) of the beat signal in the down phase, for which the presence or absence of a variation was determined by the comparison part 18, is removed by the variation removing part 21 if determined as including a variation, whereas it is stored in the storage part 20 and input to the distance and speed deriving part 22 if determined as including no variation.
Here, note that the frequency data U(txe2x88x92xcex94t) and D(txe2x88x92xcex94t) of the last beat signal may be used instead of the frequencies U(t) and D(t) of the current beat signal when the frequencies of the beat signal having been determined as including a variation are removed by the variation removing part.
The distance and speed deriving part 22 calculates the distance and the speed for the frequencies U(t) and D(t) of the input beat signal according to the expressions (5), (6).
The signal processing part of the known radar system is constructed as mentioned above, and is able to suppress variations in the beat frequencies in the time series direction. However, the prior art techniques including the above examples require a frequency pair of beat frequencies, i.e., a beat frequency in the up phase and a beat frequency in the down phase, in order to obtain the distance and the speed of a target.
Therefore, if one of the frequencies is not obtained, there will be a (non-detection) target undetected due to the fact that no frequency pair is selected even though the target actually exists. On the other hand, an incorrect or wrong frequency pair might be selected by the use of past beat frequencies instead of current beat frequencies not obtained, so that there will appear a target (false target) which can not actually exist. As a result, the reliability of the measurement results is deteriorated by these factors.
The present invention is intended to obviate the above-mentioned problems, and has for its object to provide a distance and speed measuring method and a radar signal processing apparatus using the method, which can obtain reliable measurement results while reducing false targets and undetectable targets by finding the distance and speed of a target based solely on the frequency of a beat signal of up (or down) phase by using information in a time series direction of the frequency of the beat signal of up (or down) phase.
In order to achieve the above object, a distance and speed measuring method according to the present invention, in which a relative distance and a relative speed of a target are measured based on a beat signal generated from a transmission signal and a reception signal of a continuous wave radar, which is frequency modulated by a triangular wave, is characterized by including: a present measurement stage in which beat frequencies are extracted from the beat signal in an up phase (modulation frequency increase period) and in a down phase (modulation frequency decrease period), and a frequency pair of beat frequencies corresponding to the target is selected among the extracted frequencies, the relative distance and the relative speed of the target being obtained based on the thus selected frequency pair as observed values, from which a relative distance, a relative speed and a beat frequency of the target are obtained as predicted values at the next observation time; and next and following measurement stages in which relative distances and relative speeds of the target at the next and following observation times are measured by using only beat frequencies in either one of the up phase and the down phase.
In addition, the next and following measurement stages is characterized in that priority is given to processing by beat frequencies in either one of the up phase and the down phase, and processing by beat frequencies in the other phase alone is carried out only when no target is detected in the one phase.
Moreover, the next and following measurement stages is characterized in that the observed values, the predicted values, and smoothed values which are obtained from the observed values and the predicted values are used when relative distances and relative speeds of the target at the next and following observation times are obtained by using only beat frequencies in either one of the up phase and the down phase.
Further, the next and following measurement stages is characterized in that assuming that an estimated distance value, an estimated speed value, an estimated beat frequency value in the up phase, an estimated beat frequency value in the down phase, an observed beat frequency value in the up phase, and an observed beat frequency value in the down phase, at the next observation point in time t+xcex94t, are Rp(t+xcex94t), Vp(t+xcex94t), Up(t+xcex94t)x, Dp(t+xcex94t)y, U(t+xcex94t)x, and D(t+xcex94t)y, respectively, a smoothed distance value Rs(t+xcex94t) and a smoothed speed value Vs(t+xcex94t) are calculated by using the following expression:
Rs(t+xcex94t)=Rp(t+xcex94t)+xcex1xc3x97{Up(t+xcex94t)xxe2x88x92U(t+xcex94t)x}
Vs(t+xcex94t)=Vp(t+xcex94t)+xcex2xc3x97{Up(t+xcex94t)xxe2x88x92U(t+xcex94t)x}
Rs(t+xcex94t)=Rp(t+xcex94t)+xcex1xc3x97{Dp(t+xcex94t)yxe2x88x92D(t+xcex94t)y}
Vs(t+xcex94t)=Vp(t+xcex94t)+xcex2xc3x97{Dp(t+xcex94t)yxe2x88x92D(t+xcex94t)y}
where xcex1 and xcex2 are constants.
Furthermore, a radar signal processing apparatus according to the present invention, in which a relative distance and a relative speed of a target are measured based on a beat signal generated from a transmission signal and a reception signal of a continuous wave radar, which is frequency modulated by a triangular wave, is characterized by including: frequency analysis means adapted to receive the beat signal in an up phase and in a down phase, respectively, for extracting frequencies of the beat signal; frequency pair selection means for selecting a frequency pair corresponding to the target from the frequencies of the beat signal in the up phase and in the down phase extracted by the frequency analysis means; distance and speed deriving means adapted to receive the frequency pair selected by the frequency selection means for obtaining the relative distance and the relative speed of the target at present; distance and speed prediction means adapted to receive the relative distance and the relative speed of the target at present from the distance and speed deriving means for calculating an predicted distance value and an predicted speed value of the target after a lapse of a prescribed time while assuming the movement of the target; frequency prediction means adapted to receive the predicted distance value and the predicted speed value from the distance and speed prediction means for calculating an predicted frequency value of the beat signal in the up phase or in the down phase; frequency comparison means for making a comparison between the predicted frequency value of the beat signal predicted by the frequency prediction means and the frequency thereof after a lapse of the prescribed time thereby to determine the presence or absence of a beat frequency whose difference in the above comparison result exists in the range of a preset allowable frequency width; and distance and speed smoothing means for calculating a smoothed distance value and a smoothed speed value based on the predicted distance value and the predicted speed value from the distance and speed prediction means, the predicted beat frequency from the frequency prediction means, and an observed frequency value of the beat signal after a lapse of the prescribed time obtained by the frequency analysis means; wherein relative distances and relative speeds of the target at the next and following observation times are obtained by the distance and speed smoothing means through the use of only the beat frequency in either one of the up phase and the down phase obtained by the frequency prediction means.
Still further, the radar signal processing apparatus is characterized in that the frequency prediction means, the frequency comparison means and the distance and speed smoothing means are provided in one set for each of the up phase and the down phase; at the next and following measurement times, priority is given to the processing of the frequency prediction means, the frequency comparison means and the distance and speed smoothing means in either one of the up phase and the down phase, and processing is carried out by the frequency prediction means, the frequency comparison means and the distance and speed smoothing means in the other phase alone when no target is detected in the one phase.
Besides, the distance and speed smoothing means is characterized in that assuming that an estimated distance value, an estimated speed value, an estimated beat frequency value in the up phase, an estimated beat frequency value in the down phase, an observed beat frequency value in the up phase, and an observed beat frequency value in the down phase, at the next observation point in time t+xcex94t, are Rp(t+xcex94t), Vp(t+xcex94t), Up(t+xcex94t)X, Dp(t+xcex94t)y, U(t+xcex94t)x, and D(t+xcex94t)y, respectively, a smoothed distance value Rs(t+xcex94t) and a smoothed speed value Vs(t+xcex94t) are calculated by using the following expression:
Rs(t+xcex94t)=Rp(t+xcex94t)+xcex1xc3x97{Up(t+xcex94t)xxe2x88x92U(t+xcex94t)x}
Vs(t+xcex94t)=Vp(t+xcex94t)+xcex2xc3x97{Up(t+xcex94t)xxe2x88x92U(t+xcex94t)x}
Rs(t+xcex94t)=Rp(t+xcex94t)+xcex1xc3x97{Dp(t+xcex94t)yxe2x88x92D(t+xcex94t)y}
Vs(t+xcex94t)=Vp(t+xcex94t)+xcex2xc3x97{Dp(t+xcex94t)yxe2x88x92D(t+xcex94t)y}
where xcex1 and xcex2 are constants.